The present application relates to the field of fiber optic communication and, more particularly, to optical packaging techniques and designs used for multiple wavelength transmitters.
In the past few decades, optics has gradually become the favored media for transmitting high bandwidths of information. Compared to electrical cabling, fiber optics can transmit modulated light for extreme distances with low loss and low distortion. As the bandwidth requirements in datacenters and between switches and routers have increased, optical links are becoming necessary in ever shorter domains. Thus gradually optics has migrated from long haul, to metro, and now to enterprise and datacenters. In previous decades the signal bandwidth through a fiber has increased generally by modulating the lasers faster and having higher speed photodetectors on the receiver. Thus the industry went from 622 Mbits/second to 2.5 Gb/s and then 10 Gb/s. But now it is becoming harder to have the direct line rate exceed 10 Gb/s or 25 Gb/s. Thus to get to higher speeds, it is generally necessary to put parallel channels within the same fiber, where 40 Gb/s, for example, is achieved using four lanes of 10 Gb/s.
This parallelism can be achieved in a number of ways. Most simply, one could use a ribbon fiber, where there is 10 Gb/s modulated light in each fiber. Alternatively, one could use a more advanced modulation scheme, where the signal has multiple levels, or is modulated in phase as well as amplitude thus achieving multiple bits per symbol. Perhaps the most practical way is to use multiple wavelengths of light, with each signal modulating a light beam of a different wavelength. Because the intrinsic bandwidth of an optical fiber is very high, all the different wavelengths can be multiplexed with a dispersive element such as a diffraction grating into a single fiber. At the receiver end, the wavelengths are demultiplexed and received separately using another matching grating and a photodiode array. Thus 40 Gb/s can be transmitted in four lanes of 10 Gb/s each, at four different wavelengths.
This Wavelength Division Multiplexing (WDM) approach has already been in use extensively in long haul or metro optical links. Typically 40 or 80 channels are multiplexed into one fiber. The problem with using this same technique for shorter distances is that the temperature of the lasers and the multiplexer must be accurately controlled as the optical wavelength of a laser and a multiplexer are both temperature-dependent. Typically in a semiconductor laser, the wavelength of generated light varies at about 0.1 nm per degree Centigrade. The optical passband of a wavelength multiplexer also varies with temperature, but at a slower rate of about 0.01 nm per degree Centigrade. To have 40 or 80 wavelengths all in the same fiber, within the 30 nm range than can be easily amplified using conventional erbium-doped fiber amplifiers, the wavelengths have to be closely spaced at 100 GHz (0.8 nm) or 50 GHz (0.4 nm) spacing. As the equipment temperature varies from −5 C to 75 C, without temperature control a laser would change wavelengths by 8 nm, and a multiplexer by 0.8 nm, in both cases enough to run over other channels. Thus all the optical components are carefully temperature controlled, either with heaters or thermoelectric Peltier coolers.
An alternative for smaller distance optical interconnects that eliminates the precise temperature control is to spread out the wavelength range beyond the 30 nm of an optical fiber span, reduce the number of channels, and also dramatically increase the wavelength spacing between lasers. For example, for 40 Gb/s applications, four 10 Gb/s channels are used over a 60 nm span, with wavelengths at 1270 nm, 1290 nm, 1310 nm, and 1330 nm. With 20 nm spacing, even if the output wavelength of the laser moves by 8 nm, it will not run over adjacent channels. The shift of the output wavelength of the multiplexer of 0.8 nm is inconsequential, so no cooling is necessary. However, one still has misalignment between the output wavelengths of the lasers and the passband center frequencies of the multiplexer. If the wavelengths of the laser output and the multiplexer passband center frequency are aligned at the midpoint of the temperature range, than at the low end of the temperature range, the laser wavelength is too short by 3.6 nm, and at the high end of the temperature range, the laser wavelength is too long by 3.6 nm.
To account for this variation of wavelength with temperature, multiplexers with semi-Gaussian or flat-topped passbands may be used, but such multiplexers tend to have increased insertion loss for passbands covering an appreciable portion of the wavelengths of a channel. For example, in practical implementations, the passband wavelength of the multiplexer may be “flat-topped,” allowing good multiplexing across a 2×3.6 nm or 7.2 nm temperature range. Unfortunately, when one fabricates a flat-topped multiplexer that goes from single mode inputs to a single mode output, the insertion loss is much higher than compared to a standard Gaussian multiplexer. Flat-topped multiplexers, while having a widened passband, therefore induce additional loss, which makes the transmitter inefficient and increases power consumption.